It is known that secondary treatment of wastewater for reduction of biological oxygen demand (BOD), total organic carbon (TOC), and total suspended solids (TSS) can be carried out using beds of particulate media which act as a carrier for viable bacteria performing the biological reaction. The bed may be a fixed bed, such as the sand bed of a trickling filter, or the bed may be operated in fluidized condition. The use of a fluidized bed as a biological reactor can provide a high-rate system, since extremely high concentrations of biomass (viable bacteria) can be maintained within the system. Treatment can be accomplished in less space and time which greatly reduces equipment and operating costs. One such process for fluidized bed BOD removal is described in U.S. Pat. No. 4,009,098. Comparable advantages can also be obtained for fluidized bed tertiary treatment of wastewater to remove ammonia nitrogen (nitrification), and/or to remove nitrate or nitrite (denitrification). Processes for such tertiary wastewater treatment are described in U.S. Pat. Nos. 4,009,099 and 3,846,289.
Biological fluidized bed advanced treatment of wastewater is being promoted commercially by Ecolotrol, Inc. (Bethpage, New York) and Dorr-Oliver, Inc. (Stamford, Connecticut) as the "Oxitron" biological process. It is understood that this process as applied to secondary or tertiary treatment of wastewater utilizes the teachings of the above-cited patents. According to teachings of these patents, it is important to provide means for continuous removal of excess bacterial growth during the treatment process without interrupting the process. One means proposed to accomplish this result is the provision of a mechanical agitator in the upper portion of the fluidized bed to promote the separation and removal of cellular material. The agitator, functioning as a mixer, also breaks up foam which tends to carry with it bacterial-coated media, thereby reducing but not eliminating media loss.
The loss of bacterially-coated media particles from the fluidized bed through elutriation is undesirable. It lowers active biomass concentration (MLUSS) and reduces reaction rate. With the Oxitron process, media loss occurs continually despite the operation of a mixer at the top of the fluidized bed. Therefore, for commercially practical operation of fluidized bed reactors, it has been necessary to provide solids retention tanks and solids re-entry lines to return media particles to the fluidized bed bioreactors during operation, thereby compensating for the media loss with the treated water. Despite these expedients, loss of viable bacterial cells occur to a significant extent.
Another problem encountered in such operation of bioreactors is that of periodic instability due to variations in water flow rate. The flow of wastewater is frequently subject to diurnal fluctuations over which are superimposed other erratic changes in flow rates. Thus, unstable operation may occur with considerable frequency. The result can be a channeling effect where part of the wastewater flows through the bed without being adequately treated, and/or a substantial loss of bacterial-coated media due to washout. Continual human supervision of the operation is therefore desirable.
A further limitation of fluidized bed reactors for biological wastewater treatment is that the high rate of treatment is at the expense of completeness of treatment. It is difficult to obtain as complete removal of the organic contaminants in high-rate fluidized beds as with biological treatment in the low rate non-fluidized beds of trickling filters.